The prior art hydrocyclones have, in most cases, a conical shape and are comprising an upper cylindrical and a lower conical portion. They are optimal for obtaining high mass-recoveries, whereby the discharge of solids is made by means of the underflow reject nozzle. In the desired fractionation, one obtains generally small separation meshes. If one desired an increase in the separation mesh, this was possible only by means of an increase of the inflow concentration, i.e., the percentage portion of solids in the feed suspension. This, however, is only possible with an insufficient separation sharpness. In addition, there exists, in general, the disadvantage that the concentration of the feed is continuously changed, especially in the preparation of raw material. This causes the resultant separation mesh of the discharged fractions to fluctuate over an excessively large grain range; such products having an excessively high imperfection quality are not suitable for many purposes of this application.
It should be pointed out here that for the concept of the separation mesh the expressions "cut point" or "separation cut" are common in Germany, for example, the English technical terminology, such as "separation mesh" is used for Trennkorngrosse", or "cut" size is also a commonly used term. In any case, it concerns the over-lapping point of the individual average diameter of the two particle fractions; for simplification only, the term "separation mesh" will be used in the following.
In the prior art hydrocyclones there exists an additional problem in that one is able to fractionate with these only up to a predetermined separation mesh whereby the maximum value depends on the cyclone diameter and the separation mesh barely exceeds 150 .mu.m during the utilization of single-phase-operated cyclones. Separation mesh of 200 .mu.m and more could not be reached in general. The reduction of the pressure of the inflowing suspension again is limited downwardly in order to retain the twist (pitch) which is required for functioning efficiently.